Field of the Invention
The present invention relates to a radar equipment having at least one antenna for transmission and reception of radar signals, a signal generator for supplying the antenna, and a mixer and a filter for producing a mixed signal, the signal processor receiving the mixed signal and further sensor signals, recording the frequencies of maxima in the spectrum of the mixed signal separately on the basis of a rising and a falling modulation phase as object frequencies, and calculating the distance and the relative speed of a target object from the object frequencies.
The present invention also relates to a method for operating a radar equipment which has at least one antenna for transmission and reception, a signal processor which supplies the antenna, and a mixer and a filter which produce a mixed signal and feed the mixed signal to the signal processor, wherein those digital samples of the mixed signals which are detected and recorded in each modulation cycle during the two modulation phases are subjected separately to Fast Fourier transformation in successive measurement cycles which include a rising and a falling modulation phase in the digital signal processor and a subsequent evaluation pause in order to determine the object frequencies from the maxima contained in the spectra.
The invention is used in particular for collision warning and control of the movement of vehicles.
The frequency band which is allocated for radar operation is around 77 GHz. The predetermined (technical and legal) boundary conditions for the transmitter result in the use of a frequency modulated continuously transmitted carrier wave (FMCW radar) as the only cost-effective possibility.
In the case of that modulation, the transmission frequency is normally varied within a narrow frequency band of typically 200 MHz, frequency shift, with the aid of a sawtooth or triangular-waveform modulation signal. One modulation cycle thus includes a rising and a falling modulation phase. A receiver mixes the transmitted signal which is modulated in that way with echo signals which are reflected from target objects, and mixes those echo signals to form a mixed signal which, per modulation phase and target object, contains one frequency which is characteristic of that target object and is referred to below as the target object frequency. The distance and relative speed of the target object can then be calculated from the object frequencies in the two modulation phases of a modulation cycle. The frequency which characterizes the distance in that case is proportional to the modulation rate (frequency shift per unit time). In the case of a moving target object, the object frequency is additionally dependent on the Doppler effect and is thus proportional to the speed of the target object and the transmission frequency.
Normally, the modulation rate is selected to be as high as possible in order to permit that component of the object frequency which is caused by the Doppler effect, the relative speed component fv, to be kept as small as possible in comparison with the range component fr within the detection area under consideration.
Those two components result from the object frequencies during the two modulation phases as follows: the object frequency in the first modulation phase with a rising frequency is given by the formula fu=.vertline.fv-fr.vertline., and that in the second modulation phase with falling frequency is given by the formula fd=.vertline.fv+fr.vertline.. Since fr becomes very large in comparison with fv, if the modulation rate is very high, the assignment of the object frequencies belonging to the same target object within the two modulation phases, and thus the calculation of distance and relative speed based thereon, are simple, even if a plurality of target objects are present.
In principle, the mixed signal must be sampled at a rate which is greater than the frequency resulting from the Nyquist theorem in the case of the largest detection area. Thus, systems having a modulation rate which is as high as possible require very fast and thus costly analog to digital converters in conjunction with correspondingly fast processors and memory systems for the buffer storage of the samples for further analysis. Such a system can therefore not be used for a cost-effective radar system, such as for use in a car, for example.
Admittedly, conventional digital modules and processors can be used if the modulation rate is appropriately reduced. However, that would have the consequence, in some cases (fast target objects at short range), of causing the value of fd to become greater than that of fr which results in ambiguities in the assignment of the object frequencies in the two modulation phases as soon as a plurality of target objects are present. In addition, the measurement accuracy and the resolution of the system are adversely affected as a result of the poorer separation between adjacent object frequencies and as a result of the smaller number of samples within a modulation phase.
A further limitation results from the low transmission power, with the consequence that the echo signals are very close to the system noise.